The design of an imaging device for a space observation mission depends on the objectives of the mission, as well as the orbit of the satellite or the altitude of the aircraft carrying the device.
For example, observation of the Earth from a satellite in low circular orbit can be carried out during one single pass of the satellite within several adjacent strips parallel to the direction of travel of the satellite. By low orbit is meant an orbit for which the altitude of the satellite is comprised between 400 km (kilometers) and 1,000 km. Each observation strip is called a sub-swath in the jargon of a person skilled in the art. It corresponds approximately to a row of photosensitive elements of one or more detector(s) that is (are) used at the same moment to capture the image of an across-track segment of the sub-swath.
The total scan then results from a combination of the continuous acquisition of images within along-track segments of sub-swaths, and across-track offsets of the line of sight in order to swap from one sub-swath to another. Such a scanning mode is called “push-broom”, and makes it possible to capture as an image an entire strip of the Earth's surface, called a swath, the width of which is approximately equal to that of a sub-swath, multiplied by the number of sub-swaths. In this manner, a swath width of 20 to 30 km can be obtained with a resolution on the ground (“ground sampling distance” or GSD) that is less than 1 m (meter), for example of the order of 0.5 m.
According to a first push-broom scanning mode, image acquisition for two along-track segments of sub-swaths that are covered one after another is carried out in the same direction of scanning. Said direction of scanning is thus the direction of travel of the satellite. Said first mode, known as “unidirectional push-broom” mode and shown in FIG. 1a, requires controlling rapid backward movements of the line of sight of the imaging device, between the successive acquisitions of two along-track segments of different sub-swaths.
A second scanning mode consists of covering successively acquired along-track segments of sub-swaths in opposite directions. Said second scanning mode, shown in FIG. 1b, is known as “bidirectional push-broom” mode. It makes it possible to reduce the dead time between the successive acquisitions of along-track segments of different sub-swaths, as the line of sight then describes a series of out-and-back movements parallel to the direction of travel of the satellite. Each period of dead time now corresponds only to the across-track shift of the line of sight from one sub-swath to the next, and to the reversal of the direction of scanning.
In FIGS. 1a and 1b, the references used have the following meanings:                S: satellite carrying the imaging device        100 imaging optics of the imaging device        A100 line of sight of the imaging optics        PF focal plane of the imaging optics        L rows of photosensitive elements in the focal plane        DL direction of the rows of photosensitive elements        F: swath on the surface of the earth        B: image capture scanning in the entire swath        F1-F5: sub-swaths        V: direction of scan in progress of an along-track segment of sub-swath, identified in the rest of the description by the direction of image motion in the focal plane.        
The across-track offsets of the line of sight as well as any reversals of the direction of scanning can be executed in different ways known to a person skilled in the art. For example, they can be carried out using an orientable mirror arranged in front of the input of the imaging optics 100, or by varying the attitude of the satellite S in a manner synchronized with its travel in the orbit.
FIGS. 1a and 1b also show the orientation of the rows L of photosensitive elements in the focal plane PF. For the two push-broom scanning modes, unidirectional and bidirectional, the attitude of the satellite S is adjusted so that the rows L of photosensitive elements are perpendicular to the along-track direction of the swath F, the latter being parallel to the ground track of the orbit of the satellite S. As a result, the photosensitive elements of one same row L simultaneously receive the luminous fluxes originating from point sources of one same across-track segment of sub-swath.
Apart from the resolution and the swath width, the specification for an observation mission also comprises constraints on the following aspects: the duration of accumulation for each image point, the acquisition of image data simultaneously for several different wavelengths, the cost and weight of the imaging device, the positioning in relation to each other of all the detectors used, etc.
Implementing time-delay integration (TDI) detectors makes it possible to use a size of photosensitive elements that is small enough to obtain a fine ground sampling distance, with a total integration time that is itself compatible with low luminous flux levels. But image detectors of the TDI type in principle can only operate for sub-swath scanning, which is carried out so that the image moves on the detector in the row transfer direction of TDI operation. It is therefore not possible to use a single TDI detector in bidirectional push-broom scanning mode, if said TDI detector has one single row transfer direction.
The imaging device WorldView-2 made by DigitalGlobe solves this incompatibility between the unidirectional nature of most existing TDI detectors and the bidirectional push-broom scanning mode. To this end, the WorldView-2 device uses only TDI detectors which each has two possible row transfer directions that are opposite to each other. FIG. 2 shows the arrangement of the image detectors in the focal plane PF which was adopted for the WorldView-2 device. Two parallel rows of TDI detectors that are allocated to a panchromatic imaging channel with a broad transmission window and denoted by the reference 1 are arranged in the focal plane PF of the device, with parallel row transfer directions. Each TDI detector of the panchromatic channel can therefore be activated regardless of the sub-swath scanning direction. Additional detectors 2a-2h are each formed of a row of photosensitive elements. They are also of the TDI type with two possible row transfer directions. All the detectors 2a are combined with narrow spectral width filters around a first colour, those marked 2b with even narrower spectral width filters around a second colour, and so on for the detectors 2c-2h. The WorldView-2 imaging device thus combines obtaining a polychromatic image, i.e. capture with a broader spectral width, with eight so-called chromatic channels, i.e. with smaller spectral widths than the panchromatic channel, and with a fine ground sampling distance, produced by the panchromatic channel. The base pattern of the arrangement of the detectors is contained within the box in broken lines C. It is repeated along the direction of the rows of photosensitive elements, in order to increase the sub-swath width. The overlaps in the direction of the rows for detectors of the same type, and their offset in the perpendicular direction, make it possible to overcome a difficulty in respect of the space requirement within the focal plane PF, and to join the segments of image lines that are captured by different detectors.
Due to the row transfer in two directions that is used for the TDI detectors 1 of the panchromatic channel, each detector is produced on a different substrate, separate from that of the neighbouring detectors 1 or 2a-2h. Each detector 1 has two transfer registers and corresponding arrays of outputs of the detection signals that are situated at opposite ends of the columns of photosensitive elements of said detectors. The resulting space requirement for the connections of outputs of the detectors 1 of the panchromatic channel then prevents the production of one of said detectors 1 on a substrate common with some of the detectors 2a-2h of the chromatic channels.
For this reason, the WorldView-2 device comprises a very large number of separate substrates of image detectors, the arrangement of which in the focal plane at precise locations is particularly time consuming. Moreover, the relative positions of some of the substrates can vary unintentionally, thus producing errors of alignment between the images captured by the detectors.
In other imaging devices of different design, the arrangement of separate detectors in different focal planes requires spatially dividing the image formation beam produced by the imaging optics. Such a design with beam division is more complex, because it requires using additional optical components, and the weight as well as the space requirement of the imaging device are increased in consequence.